Device for the indirect heating of air

ABSTRACT

A device for the indirect heating of air or another gas at high pressure to a high temperature, consisting of a heat exchanger of a special construction submerged in a fluid bed of the &#34;classical type&#34;, through which flows a fluidized hot, finely powdered bed material which is introduced (2) below the bed surface (3) and discharged through an overflow (5). The heat exchanger surface is formed by hairpin-shaped tubes (6) with legs of unequal length, the shorter legs thereof discharging into the upper bottom of an inlet chamber (7) and the longer legs thereof extending down through a tubular lead-through (9) between the bottoms of said inlet chamber (7), which lead-through is concentric with said tube leg and opens into the upper bottom of an outlet chamber (8), with an annular channel between said tube (6) and said lead-through (9). A bell (12) is secured to each of the long legs above the lead-through (9), forming together with the annular channel (11) a distribution means of &#34;bubble cap&#34;  type for the fluidizing air supplied to the fluid bed (13). The low space (14) between the pressure chambers is separated from the fluid bed and is supplied with a small air flow, which through the annular channels (11) bubbles out below the &#34;bells&#34; (12) and fluidizes the bed (13). The invention is also related to the use of said device for the indirect heating of gas, especially air.

Indirect heating of large flows of air with high pressure (10-30 bars)to high temperature (750°-850° C.) is of interest in connection withpower generation based on solid fuels, especially coal, comprisingcombinations of gas and steam turbines in purpose of reaching highefficiency.

The present invention relates to a device advantageous for said heatingcomprising a specially designed heat exchanger submerged in a fluid bedof classical type through which fine-grained hot bed material, as anexample coal ash, is flowing, which material has been heated to a hightemperature by combustion of pulverized coal in mixture with the bedmaterial.

The fluidized bed is maintained at 0.6<ε<0.7 (ε=voidage) in a chamberwith a preferably rectangular horizontal section, which is supplied withhot bed material, preferably below the bed surface, in the vicinity ofone of the chamber walls and from which chamber the bed material flowsover a weir in the opposite chamber wall.

According to the invention the heat exchanger surface consists of anadapted number of hairpin bent tubes, which for reason of strength andcost preferably have an inner diameter of 20-80 mm. The "hairpins" havestraight vertically downward pointing legs of differing length which arefastened to separate disc shaped pressure chambers--one upper inletchamber and one lower outlet chamber--each with horizontal betweenthemselves connected plane parallel walls and sides with preferably halfcircular section. The short leg of the "hairpins" opens into the upperwall of the inlet chamber while the long leg extends down through a tubeshaped conduit between the horizontal walls of the inlet chamber andopens into the upper horizontal wall of the outlet chamber.

According to the invention the tube shaped conduit is fastened to thehorizontal walls of the inlet chamber concentrically with the long legand has such a length that it extends to an adapted level above theupper horizontal wall of the inlet chamber and has such an innerdiameter that an annular channel with suitable width is formed betweenthe tube leg and the conduit. The latter acts simultaneously as afortifying element between the horizontal walls of the inlet chamber.

Another characteristic feature of the device according to the inventionare bubblecaps with a suitably toothed lower edge which are fastened-tothe long leg at an adapted level above the upper edge of the conduits.Together the bubblecaps and the annular channels form suitabledistributors for the feeding of the fluidizing air to the fluid bed inwhich the heat exchanger is submerged.

The device according to the invention has a low space between thepressure chambers separated from the fluid bed preferably by a "skirt"that extends from the sides of the inlet chamber past the sides of theoutlet chamber and ends somewhere at approximately the level of thelower horizontal wall of the outlet chamber. The fluidizing of the bed,in which the heat exchanger is submerged, is done with a small adaptedflow of gas that is fed into the space between the pressure chambers andflows up through the annular channels and out below the toothed loweredge of the bubblecaps, thereby fluidizing the bed.

A simple way of feeding a fluidizing flow of air to the space betweenthe pressure chambers is by tapping an adapted flow from the inletchamber through one or more small holes with adapted diameter in thelower horizontal wall of the chamber.

Because of the high temperature of the bed material (>800° C.) and theoutlet air together with the high pressure, a large part of the tube'ssurface and the whole outlet chamber reaches a high temperature. Thismeans that only walls made of heat-resistant alloys, i.e. Incoloy 800 H,match the strains with reasonable wall thickness.

By using tubes with comparatively small diameter (20-80 Mm) the wallthickness will be very reasonable, which is of importance because of thehigh specific material cost.

The outlet chamber, where the hot air from the "hairpins" is collected,is like the inlet chamber disc shaped with plane parallel horizontalwalls (bottoms) and sides with half circular section, which enablesfortifying the walls between themselves with an adapted number offortifying elements and as tubes, bolts etc. For welding reasons, tubeshaped fortifying elements are preferred, whereby the fortifyingelements at the center of (in line with) the hairpin tubes have a largerdiameter than the outer diameter of the hairpin tubes and are perforatedwith several holes, the total openings of which is >2 times the tubesection area.

The pressurized air entering the inlet chamber is according to theinvention piped vertically down through the bed, which pipe preferablyopens into the center point of the upper horizontal wall of the inletchamber. Because the air after the compressor has a reasonabletemperature (300°-400° C.) the wall temperature of the feeding pipe willbe comparatively low and the necessary wall thickness moderate.

The hot pressurized air in the outlet chamber is preferably tappedthrough a vertically downward pointing pipe centrally fastened to thelower horizontal wall of the chamber. The volume flow here is as aconsequence of the high temperature almost double the feed flow and thepipe diameter must be relatively large in order to get reasonable gasvelocity. By making the pipe according to the invention with a doublewall, an outer pressure resistant wall manufactured of normal pressurevessel material and an inner thin wall manufactured of heat-resistantmaterial and flush the space between the walls with pressurized coolingair, the problem can be solved. The heated cooling air can for instancewith advantage be used as combustion air.

In order to further elucidate the invention an embodiment thereof is nowdescribed in connection with FIG. 1:

The device according to FIG. 1 comprises a fluid bed chamber 1 withrectangular horizontal section in which a fluidized bed 13 is maintainedat 0.6<ε<0.7, the bed material of which essentially consists of finelydivided coal from combustion of pulverized coal and which is fed throughan inlet tube 2 into the bed 13 at 4 below the surface 3 of the bed andleaves the bed over the weir 5.

In the fluidized bed 13 a heat exchanger is submerged. Its heatexchanging surface is made of an adapted number of "hairpin" bentvertical tube elements 6 with legs of different length fastened in andopening into separate disc shaped pressure chambers--an upper inletchamber 7 and an outlet chamber 8 situated just below--each with planeparallel horizontal walls fortified between themselves with tube shapedfortifying elements 10 and sides with half circular section. The"hairpin" tubes' 6 short legs open into and are welded to the upperhorizontal wall of the inlet chamber while the long legs extend throughtube shaped conduits 9 welded between the horizontal walls of the outletchamber 7, whereby the long legs open into and are welded to the upperhorizontal wall of the outlet chamber 8. The conduits 9 have such lengththat they extend to an adapted level above the upper horizontal wall ofthe inlet chamber 7 and such inner diameter that annular channels 11with adapted width are formed between the long tube legs and saidconduits.

At adapted level above the upper end of the conduits 9 a bubblecap 12with toothed lower edge is fastened on every long leg. These caps formtogether with the annular channels 11 the distributor for the fluidizingair to the bed 13, which is tapped from the inlet chamber 7 throughholes 17. In order to keep the space 14 between the pressure chambersfree from bed material the space 14 is separated from the fluidized bed13 by the "skirt" 15.

In order to withstand the strains, that the pressure chambers areexposed to by the high pressure and high temperature, the horizontal(disc shaped) walls of the pressure chambers are fortified betweenthemselves by fortifying elements 21 shaped as perforated tubes at thecenter of (in line with) the tube leg openings and as bolts or tubeselsewhere. The sides of the pressure chambers have half circular sectionin order to withstand the strains.

The supply of pressurized air 22 to the heat exchanger, usually with atemperature of 300°-400° C., is done through a vertical pipe 23 downthrough the bed 13, said tube opening centrally into and being welded tothe upper horizontal wall of the inlet chamber 7. The discharge of hotair from the outlet chamber 8 is done through a vertically downwardpointing centrally placed pipe 18 welded to the lower horizontal wall 16of the chamber. The pipe has a double wall 20+19 with cooling by passingpressurized air between the walls.

We claim:
 1. A device for indirect heating of air or other gases of highpressure to high temperature particularly suitable for large flows,comprising a heat exchanger of special design submerged in a fluidizedbed of "classical" type, through which finely comminuted bed material isflowing, which bed material, as an example, has been heated bycombustion of coal in mixture with the bed material, wherein:a) thefluidized bed is maintained in a chamber which is supplied with hot bedmaterial below the bed surface and from which chamber the bed materialflows over a weir; b) the heat exchanger surface consists of an adaptednumber of hairpin bent tubes with vertical downward pointing legs ofdifferent length opening into and fastened to separate disc shapedpressure chambers--an upper inlet chamber and an outlet chamber situatedjust below--each with plane parallel horizontal walls fortified betweenthemselves by fortifying elements and sides with half circular section,whereby the short leg of the hairpin tubes opens into the upperhorizontal wall of the inlet chamber while the long leg extends downthrough a tube shaped conduit between the horizontal walls of the inletchamber and opens into the upper horizontal wall of the outlet chamber;c) the tube shaped conduit of the inlet chamber is fastened between thehorizontal walls of the chamber concentrically with the long leg of thehairpin bent tube extending therethrough and has such length, that itextends above the upper horizontal wall of the inlet chamber and hassuch inner diameter, that an annular channel with adapted width isformed between the tube leg and the conduit; d) a bubblecap is fastenedon every long leg at an adapted level above the upper end of theconduit, thereby together with the annular channel forming a suitabledistributor of bubblecap type for the fluidizing air of the bed in whichthe heat exchanger is submerged.
 2. A device according to claim 1,wherein the low space between the pressure chambers is separated fromthe fluidized bed by a "skirt" extending down from the sides of theinlet chamber past the sides of the outlet chamber ending at about thelevel of the lower horizontal wall of the outlet chamber and wherein asmall adapted flow of gas is fed into the space, which flow via theannular channel bubbles out below the bubble-cap and fluidizes the bed.3. A device according to claim 2, wherein the plane horizontal walls ofthe pressure chambers are fortified between themselves with fortifyingelements, whereby elements at the center of the hairpin tubes are tubeshaped with an inner diameter larger than the outer hairpin tubediameter and with perforated jacket, the total area of the holes beinggreater than 2 times the section area of the hairpin tube.
 4. A deviceaccording to claim 3, wherein the hot air is tapped from the bottom ofthe outlet chamber through a downward pointing pipe, which has an outerpressure resisting wall and an inner thin wall of heat-resistantmaterial, whereby the space between the walls is flushed withpressurized cooling air.
 5. A device according to claim 2, wherein thehot air is tapped from the bottom of the outlet chamber through adownward pointing pipe, which has an outer pressure resisting wall andan inner thin wall of heat-resistant material, whereby the space betweenthe walls is flushed with pressurized cooling air.
 6. A device accordingto claim 2, wherein the small flow of gas fed to the space between thepressure chambers is tapped from one or several small holes with adapteddiameter in the lower horizontal wall of the inlet chamber.
 7. A processfor the indirect heating of air which comprises the use of a device asdefined in claim 2 for indirect heating of air or other oxidizing gas toa high temperature greater than 700° C.
 8. A process according to claim7, wherein the air is heated from the compressor of a gas turbine beforeit is fed to the expansion turbine of a gas turbine.
 9. A deviceaccording to claim 1, wherein the plane horizontal walls of the pressurechambers are fortified between themselves with fortifying elements,whereby elements at the center of the hairpin tubes are tube shaped withan inner diameter larger than the outer hairpin tube diameter and withperforated jacket, the total area of the holes being greater than twotimes the section area of the hairpin tube.
 10. A device according toclaim 3, wherein the hot air is tapped from the bottom of the outletchamber through a downward pointing pipe, which has an outer pressureresisting wall and an inner thin wall of heat-resistant material,whereby the space between the walls is flushed with pressurized coolingair.
 11. A device according to claim 9, wherein the small flow of gasfed to the space between the pressure chambers is tapped from one orseveral small holes with adapted diameter in the lower horizontal wallof the inlet chamber.
 12. A process for the indirect heating of airwhich comprises the use of a device as defined in claim 9 for indirectheating of air or other oxidizing gas to a high temperature greater than700° C.
 13. A device according to claim 1, wherein the hot air is tappedfrom the bottom of the outlet chamber through a downward pointing pipe,which has an outer pressure resisting wall and an inner thin wall ofheat-resistant material, whereby the space between the walls is flushedwith pressurized cooling air.
 14. A device according to claim 13,wherein the small flow of gas fed to the space between the pressurechambers is tapped from one or several small holes with adapted diameterin the lower horizontal wall of the inlet chamber.
 15. A process for theindirect heating of air which comprises the use of a device as definedin claim 13 for indirect heating of air or other oxidizing gas to a hightemperature greater than 700° C.
 16. A device according to claim 1,wherein the small flow of gas fed to the space between the pressurechambers is tapped from one or several small holes with adapted diameterin the lower horizontal wall of the inlet chamber.
 17. A process for theindirect heating of air which comprises the use of a device as definedin claim 16 for indirect heating of air or other oxidizing gas to a hightemperature greater than 700° C.
 18. A process according to claim 17,wherein the air is heated from the compressor of a gas turbine before itis fed to the expansion turbine of a gas turbine.
 19. A process for theindirect heating of air which comprises the use of a device as definedin claim 1 for indirect heating of air or other oxidizing gas to a hightemperature greater than 700° C.
 20. A process according to claim 19,wherein the air is heated from the compressor of a gas turbine before itis fed to the expansion turbine of a gas turbine.